I. Field of the Invention
The present invention relates generally to engine fuel systems and, more particularly, to a fuel system for a turbine engine.
II. Description of the Prior Art
In certain types of turbine engines, such as turbine engines used in cruise missile systems, the engine is stored for long periods of time without use and yet must be ready for subsequent launch without special maintenance, handling or launch preparation. Furthermore, the cruise missile and its engine are operated only a single time which terminates in the destruction of the missile. Consequently, it is particularly important that the cost associated with the engine installation, maintenance and handling be minimal but, at the same time, enjoy a very high system reliability.
Previously, it has been difficult to obtain a low cost and yet highly reliable fuel system with respect to the simplicity of the initial installation of the fuel system, the simplicity of the fuel system during operation and also the sequencing for engine start up. In particular, it is highly desirable that the engine be capable of installation in the cruise missile with only a single fuel connection between the missile air frame and the engine fuel system. The fuel tank, of course, is conventionally contained within the missile air frame. In addition, it is highly desirable that no special installation operation, such as purging the air from the fuel system, be required after the installation of the engine in the air frame.
Additionally, in a cruise missile system, it is necessary that the turbine engine reach its maximum engine thrust in an extremely short time after the point of launch of the missile. In order to achieve this, the engine and air frame fuel systems, lines fittings and fuel handling components must be completely filled and purged of air prior to engine starting upon launching of the missile. Unless the air is purged from the fuel system, an engine flame out can occur. If engine flame out occurs after the launch of the cruise missile, it would result in the premature destruction of the missile.
In order to prevent an engine flame out, the fuel must reach the turbine combustor at the time that the engine start and ignition sequence begins. The time available for fueling and purging air from the engine fuel system, however, is usually less than a few seconds and with many types of fuel pumps, such as positive displacement pumps of hydromechanical systems, it is difficult to meet this requirement and at the same time minimize the cost of the engine. Complex fuel systems are further disadvantageous in that such systems oftentimes include flow restrictions in which air pockets can form. Such entrapped air is later released as bubbles into the fuel flow which can shut down the engine. In addition, the fuel pumping rate of these previously known systems during engine start is generally inadequate to fill the system in sufficient time for fuel to reach the turbine combustor at the initiation of engine ignition.